It is known to fit such aircraft with actuators for eliminating or attenuating the aircraft vibration that results in particular from the alternating forces applied to the aircraft fuselage by the lift and propulsion mechanism of the aircraft (in particular its rotor).
For this purpose, aircraft vibration is measured using at least one accelerometer that delivers a vibration measurement signal, which measurement signal is processed by processor means, possibly including an algorithm recorded in an electronic card or in an on-board computer, so as to obtain a control signal that is applied to the actuator in order to generate forces opposing the measured vibration, as described in U.S. Pat. No. 5,473,698, for example.
The forces can be the result of beating motion of a mass mounted to move in translation relative to a structure of the aircraft and driven by a linear electromagnetic actuator; it is also known to use a rotary actuator comprising unbalanced rotors for producing these forces by the centrifugal effect, as described for example in U.S. Pat. No. 5,005,439.
The present invention applies in particular to vibrators comprising two pairs of unbalanced rotors (or flyweights) in which the rotors in each pair are contrarotating and synchronous, the axes of rotation of all four rotors being parallel, the (flat) rotors or flyweights extending substantially in a common plane orthogonal to the axes of rotation of the rotors.
The permanent synchronization of the two contrarotating rotors in each of the pairs of rotors can be obtained electronically; this applies to vibrators having an electric motor associated with each rotor for driving it in rotation, such as the vibrators described in the following patents: FR 2 606 110; EP 0 505 976; and EP 1 439 010. In document FR 2 606 110, a Kalman filter receives a vibratory signal delivered by an accelerometer and in turn delivers a signal representative of the frequency, the amplitude, and the phase of the vibratory system comprising the vibrating aircraft and the vibrator (or “anti-vibrator”). On the basis of this signal, a computer controls the speed and the phase of each motor, via a comparator. In document EP 0 505 976, it is stated that the Kalman filter produces an estimate of the force to be compensated and of its angular frequency, and also of a force in quadrature. That data is delivered to a calculation unit which deduces therefrom the value of a half phase-offset from which four speed setpoints are calculated that are transformed into four power supply voltages. A calculation module also makes use of the signals coming from the four angle encoders to produce a signal that is an image of the resultant force generated by the flyweights. This signal is summed with the residual acceleration signal and the result of the summing is applied as an input to the Kalman filter.
The system described in EP 1 439 010 comprises, for each unbalanced wheel, a sensor delivering a pulse signal on each revolution of the wheel whenever the flyweight is in a characteristic position, and it includes a signal processor receiving a setpoint signal. A phase reference pulse is produced during an (upward) zero crossing of the setpoint signal. On the basis of these signals, a speed error signal and a phase error signal are calculated for each rotor; furthermore, two phase setpoint signals are calculated from the normed amplitude of the setpoint signal. On the basis of these signals, a speed control signal and a phase control signal with proportional and integral correction are summed in order to provide a control signal for each motor.
Those generators and their motor control systems are complex.
The rotors in each of the pairs of rotors can also be synchronized mechanically by means of gearing as described in particular in patent EP 0 840 191, or by means of a belt as described in particular in international application WO 2004/087343.
In those vibration generators, a first electric motor is used to drive the rotors in rotation, while a second electric motor is used to adjust/drive a phase shifter mechanism and/or a differential mechanism serving to adjust mechanically a phase difference between a first pair of synchronous rotors and a second pair of synchronous rotors.